1. Field of the Invention
The present invention relates to a composite metal foil and a method for producing the porous metal foil.
2. Background Art
In recent years, lithium-ion secondary batteries and lithium-ion capacitors have been drawing attention as electric storage devices for use in mobile electronic devices such as mobile phones or laptop personal computers, electric vehicles, and hybrid vehicles. As anode current collectors for such electric storage devices, porous metal foils are used or are being considered for use. This is because making the foil porous provides benefits such that the volume or weight of the foil can be reduced (to improve fuel consumption in automobiles), that adhesive power of an active material can be improved by anchoring effect making use of the pores, and that pre-doping of lithium ions (e.g., vertical pre-doping) can be efficiently conducted by making use of the pores.
Known methods for producing such porous metal foils include (1) a method of masking the surface of a substrate in a desired pattern with an insulating film, onto which electrolytic plating is conducted to form pores in accordance with the pattern; (2) a method of providing the surface of a substrate with a specific surface roughness or a specific surface condition, onto which electrolytic plating is conducted to control nucleation; (3) a method of perforating a non-porous metal foil by etching or machining; and (4) a method of forming a three-dimensional network structure by techniques for producing metal foams or plating nonwoven fabrics.
In particular, various techniques have been proposed for the above method (2) since its steps are relatively simple and suitable for mass production. For example, Patent Literature 1 discloses a method for producing a fine-porous metal foil by subjecting a cathode having a surface roughness Rz of 0.8 μm or less to electrolytic plating. Patent Literature 2 discloses a method comprising forming an oxidized film on the surface of a cathode body made of titanium or a titanium alloy by anode oxidation method; electro-depositing copper on the surface of the cathode body to form a porous copper foil; and peeling the foil from the cathode body. Patent Literature 3 discloses a method for producing a porous metal foil provided with an aluminum alloy carrier, comprising forming even projections by etching aluminum; and gradually growing metal particles from the projections as cores for electro-deposition so as to connect the metal particles.
However, the actual situation is that it is not easy for these conventional production methods to produce a foil having a stable aperture ratio in a cost effective manner for reasons such that these production methods generally require a large number of steps and thus have a tendency of increasing production cost, that machining such as punching causes burr, and that anode oxidation process leads to difficulty in controlling nucleation. In addition, a long foil is difficult to produce, and the anode oxidation process had problems with the peelability of the porous film and the stability of the aperture ratio, in that continuous peeling of the foil destroys the oxidized film. In particular, a porous metal foil having a high aperture ratio, which is devoid of burrs and is capable of having pores made smaller, is required for anode current collectors of electric storage devices, such as lithium-ion secondary batteries and lithium-ion capacitors, with improvement in performance.
On the other hand, it is known that battery properties are improved by applying a primer to a metal foil serving as a current collector. For example, Patent Literature 4 discloses that the surface of a current collector is coated with a primer containing lithium polysilicate and, optionally, a carbonaceous component. Patent Literature 5 discloses that an electrically conductive auxiliary agent comprising one or more of a carbon powder, a carbon fiber and an electrically conductive polymer and the like is fixed on a metal foil serving as a current collector substrate, or a planar member such as a metal mesh or punching metal, using a binder.